Areas of open and inland waters and shorelines are with increasing frequency being polluted by the spillage of oil. One major approach to alleviating this problem has been the development of apparatus and methods for preventing the spreading of spilled oil by confining it to the area near its discharge. This facilitates removal, by causing the oil to concentrate in thicker layers on the surface of the water. Containment of spilled oil employs commercially available floating booms such as exemplified in the oil collection boom of U.S. Pat. No. 3,679,058 (Smith) or sorbent barriers such as that described in U.S. Pat. No. 3,739,913 (Bogosian).
Such containment techniques are not fully effective, as water currents, waves and wind conditions often lead to the escape from the containment region of oil which forms an outwardly spreading film that diminishes from a few millimeters down to fractions of a micron (0.001 mm) in thickness as it moves towards the shoreline, where it can present a serious ecological threat to sensitive areas, such as fish-hatcheries, wildlife preserves, harbours and beaches.
A number of sorbent materials have been proposed or made commercially available for the purpose of picking up oil which has escaped from containment booms and for protecting shorelines. These range from natural products, such as straw or sawdust, to synthetics such as polypropylene or polyurethane foam, or relatively thick (5 mm or greater) mats, sheets or rolls of melt-spun and fabric-stitched oleophilic polymers. Examples of the latter include 3M.TM. Type 156 pads and ALSORB.TM. sheets or rolls.
Fibrous synthetic sorbent sheets of the kind presently in use have high sorption capacities, but tend to be considerably more expensive than natural or inorganic products. Their utilization has been restricted to smaller spills in sheltered waters or to recover oil whose access by skimmers or other mechanical means is difficult, such as under docks, etc. They can as well be of value for use and re-use to pick up oil within containment areas, where the high thickness of oil enables economical use of their full capacity. Re-useability of such materials, by squeezing oil from the porous interior, is stated to be a major advantage. As a practical matter, however, reusability may be an economic necessity because of the price of such materials. Further, reusability requires apparatus, facilities and man-hours for extracting and collecting the oil from the used sorbent. Moreover, the re-use of sorbents may be prohibited by regulation in some jurisdictions, to the extent that it presents a risk of re-contamination.
Known oil sorbent materials have not adequately met the need for a relatively inexpensive sorbent which exhibits a high pick-up ratio for oil, which can be conveniently stored in quantity in a relatively small space, and which can be readily and rapidly deployed from a compact unit onto sensitive areas to protect them against approaching oil slicks.
The minimum thickness of conventional synthetic sorbent materials in the form of sheets or rolls of non-woven polymer fibres is around 5 mm, which imposes a limit on the quantity of such bulky sorbent which can be stored in a given space. As discussed in greater detail below, it is desirable that there be available for use considerably thinner sorbent materials which nevertheless exhibit an adequate affinity for oil, to make it practical to keep substantial amounts of sorbent in storage at the ready for use in the event of an emergency, and that the deployment of such materials require no complex apparatus or technique calling for special training or skill.
A principal desirable property not exhibited by presently available sorbents is the combination of a high product density (for easier storage and handling) with a sufficiently high pick-up ratio. By "pick-up ratio" is meant the mass of oil which adheres to a given dry mass of sorbent, in use, symbolized hereinafter as m.sub.0 /m.sub.s. By "product density", is meant the mass of dry sorbent contained in a unit volume of the product, as stored in preparation for use, symbolized hereinafter as m.sub.s /V.sub.s. The product of these two quantities, m.sub.0 /V.sub.s, is a useful measure of the practical efficiency of a sorbent product, being the amount of oil which can be picked up by a given volume of sorbent, and is hereinafter referred to as the "pick-up density".
Particularly on application to very thin water-borne oil films, sorbents in the form of mats or pads of synthetic fibre are subject to varying degrees of undesirable water pick-up as a portion of these thicker materials rides below the oil surface and in the water.
I have discovered that relatively thin flexible sheets of oleophilic, hydrophobic substrates, such as polyethylene film, may be used as the basis for sorbent media that are effective and commercially advantageous in protecting shoreline water or land areas from approaching oil spills, particularly for deployment on sensitive areas as a first line of defense against approaching oil slicks, but also, in particular embodiments of the method of my invention, for early assistance by delaying the spreading of oil on water on or on land by deployment near the source of an oil spill or leak. Such sorbent media are sufficiently inexpensive that disposal of the oil-charged medium after use and removal, as by incineration, is economically more practical than in the case of the more expensive synthetic sorbent media currently available. However, the sorbent media disclosed herein for use according to the invention may alternatively be re-used following extraction of absorbed and adsorbed oil. Too, having no fibrous structure and being made of thin buoyant material, such sorbent media are inherently free of water pick-up.
The underlying principle of the novel sorbent media used in my invention is that an oleophilic, hydrophobic film such as thin polyethylene film preferentially adsorbs oil which covers the surface of the film by a process of planar migration of oil across the two principal surfaces of the film. The degree of sorption of the oil on an oleophilic surface is a function of the surface area and configuration of such media. Use of the thinnest substrate permitting sufficient mechanical strength in use results in the maximized value of m.sub.0 /m.sub.s.
In this respect, the sorbent media used in my invention differ fundamentally from the fibrous web-type sorbents which, in many applications, have a substantial unused capacity for oil, as discussed below.
A further fundamental difference arises from the thinness of my film-based sorbent media allowing them to be stored for use in a fan-folded, rolled-up or other configuration having a high product density, m.sub.s /V.sub.s, so that the capacity for oil per unit volume of the sorbent as stored is advantageously large.
A helpful way of viewing the methods of my invention, in which a thin flexible sheet sorbent material of the kinds discussed is deployed from a compact supply (in a pre-deployment condition of high storage density) onto the area to be protected is that the method transforms the sorbent from a pre-deployment condition of high density to a post-deployment, in-use condition of bulk capacity for oil.
According to one embodiment of the invention, the substrate used comprises a pair of oleophilic, hydrophobic films joined together along sealing seams defining a plurality of interior pockets, each film of the sorbent material having a plurality of small apertures therethrough to admit the migration of contaminating oil between the surfaces of the films and into the pockets. This sorbent has a dilatable configuration and a specific affinity for oil, so that the oil itself acts to expand the capacity within the sorbent for oil. When a length of sheet-form sorbent of this construction is unwound tangentially axially from a closely-wound storage roll and deployed flat onto the area to be protected, incoming oil migrates through the small apertures and along the internal oleophilic surfaces of the pockets, by planar migration, gradually filling the pockets and resulting in an enhanced degree of oil pick-up.
According to a second particular embodiment of the invention, the conversion of the substrate on a roll from its pre-deployment condition of high storage density to a deployed in-use condition of bulkiness and oil capacity is effected by playing the sorbent off the roll axially so that it forms a continuous, generally helical "belt" that affords a substantial barrier to an advancing oil spill.
By contrast, products such as the 3M.TM. absorbent pads have a deployment (in-use) volume which is essentially unchanged after sorption of oil from the pre-use (storage) volume of such materials, so that the pick-up density is limited. It is the object of the present invention to provide a method of using ultra-thin carriers having a maximal storage and pre-deployment density and minimum bulk in use which is close to that of the constituent oleophilic, hydrophobic material, allowing rapid deployment requiring no special skills and which, upon deployment, presents a contoured form having effective sorptive and barrier properties for oil.